This invention cogenerates liquid transport fuels and electric power from a gasifiable material such as coal, in an environmentally acceptable and cost effective manner. This objective meshes exactly with the immediate needs to reduce dependency on imported petroleum products, which are causing serious problems to the balance of trade of the United States, and also, to reduce air pollution in general and particularly from coal-burning utilities which are suspected of causing "acid rain."
Petroleum imports reduction will be accomplished through the substitution of clean synthetic fuels from coal for transport (and other uses) in place of petroleum products, and clean electric power will be accomplished through the reduction of SO.sub.2 and NO.sub.x emissions. These accomplishments result from the separate production of gases and vapors through pyrolysis of coal which are directed through a first hot-bed cleanup system which in turn causes the molecular rearrangement of the gases and vapors yielding substantially H.sub.2 and CO and thus produce a clean feedstock syngas which is synthesized into clean fuels such as methanol, and the separate production of a low Btu gas through char gasification with air which is directed through a second hot-bed cleanup system to produce a clean low Btu gas which is burned in a power cycle to efficiently cogenerate clean electric power with the emission of very low levels of NO.sub.x and SO.sub.2. This invention is applicable to new "grassroots" facilities as well as to the repowering of existing power plants.
This invention relates to the cogeneration (coproduction) of clean liquid fuels for transport and other uses as well as clean electrical energy from gasifiable materials such as coal. Other materials may also be used. By way of example, reference will be made to coal in the following description, to denote a gasifiable material; this shall not be construed as a limitation but as a convenience for explanation purposes. The cogeneration of liquid fuel for transport and other uses, and electric power are conducted in such a way as to be economical and environmentally acceptable. The United States and other coal producing nations must reduce their dependency on imported petroleum, and further, clean electric power must be produced to maintain modern life. For example, U.S. Department of Energy Assessment dated January 1988 and Solicitation Number DE-PS01-88FEb1530 addresses the dependence on foreign oil and its inherent dangers to this country and the Solicitation stresses the specific objective of solving emissions of SO.sub.2 and NO.sub.x from existing coal-burning facilities.
This invention specifically addresses the needs for both clean liquid fuel such as methanol and clean electric power; it is configured to convert the volatile content of coal to a clean feedstock suitable for conversion to chemicals including liquid transport fuel such as methanol, and the residual char to a clean fuel gas under pressure suitable for efficient and clean electrical power production with very low levels of NO.sub.x and SO.sub.2 emission.
A number of distinct approaches to the production of liquid fuels and electrical power from coal have been developed but all of these approaches except for one which shall be described hereinafter, either address the production of liquid fuels per se or the generation of electrical power per se.
There are three processes for the production of liquid fuels from coal. The first process is the feed of syngas (CO+H.sub.2) derived from gasification of coal with relatively pure oxygen, into a synthesis loop. This process is expensive as there is the need for an oxygen plant and a shift reactor. The second process is the recovery of hydrocarbon liquids recovered from coal by coking. These liquids require cooling for the separation of condensables from non-condensables and must then be upgraded by the addition of hydrogen. The bulk of these liquids have been declared to be hazardous, particularly benzene. Further, during the separation of the condensables from the non-condensables which is accomplished by quenching with water, a very serious water pollution problem is created. The third process is concerned with the direct conversion of the organic content of the coal into liquid hydrocarbons by heat, pressure and the reaction with hydrogen. The recovered liquids are then treated again with additional hydrogen to yield a fuel product. This process is very expensive, operates at very high pressures (circa 2500 psi), requires sources of hydrogen and produces a by-product which is suspected of being hazardous.
There are three processes for the production of electric power from coal. The first is the combustion of coal in a boiler to support a steam cycle turbo-generator. Limestone scrubbing is used to remove sulfur from the flue gas, thus requiring disposal of sludge. The second is a fluidized bed which also uses limestone and generates solid waste. The third is the integration of coal gasification to combined cycle; in this process, the coal is converted to gas by partial oxidation, and the gas is then used as a fuel in a gas turbine to produce power while the hot turbine exhaust gases are used to raise steam for additional power generation.
The process referred to above for the coproduction of both methanol (liquid fuel) and electric power is the process described in the Electric Power Research Institute publication entitled Coal Gasification Systems: A guide to Status, Applications, and Economics, Project #2207, dated June 1983, pages 5-153, 5-154, 5-155 and 5-156. As is described in this reference, the once-through reaction system to make methanol from coal consists of positioning in series the gasification plant, the methanol reactor and the power generation equipment. Initially the coal is gasified at high pressure (850 psi) with oxygen and steam, the gas is cooled, desulfurized and then passed as a syngas through the methanol reactor for a single pass. For this reason the process is called "Once-Through" to convert some of the syngas into methanol. The rest of the syngas is used to generate power.
The disadvantages of the Once-Through method referenced are the following:
1. The yield of methanol is quite low because the proportions of the H.sub.2 and CO in the gas are not conducive to give a high yield as the requirement for a maximum yield is a ratio of H.sub.2 to CO of +2 (see line 17, first paragraph, page 5-153). As stated on top of page 5-154, the gas is inherently high in carbon monoxide.
2. The method requires an oxygen plant. This leads to a very expensive cost increment to the coal gasification plant. Further, when gasifying with oxygen, steam, must be injected through the tuyeres in the reaction zone in order to moderate the temperature in this zone. Therefore these requirements add to the cost of the gas produced.
3. The medium Btu gas (syngas) produced and which is used for the electric power generation (combined cycle) produces excessive amounts of NO.sub.x (a serious polluter). Water or steam must be injected in the combustion chamber to maintain the combustion temperature relatively low in order to keep the formation of NO.sub.x at a minimum. Such injection of water or steam lowers efficiency and increases costs and wastewater.
4. The method creates an incompatibility of pressures which increase capital costs. To make methanol a pressure of about 850 psi is required to convert syngas (2H.sub.2 +CO) to methanol. To generate power, a maximum pressure of only 350 psi is required when using combined-cycle electric power generation. With today's knowledge and in today's environment, combined cycle power generation is the most efficient way of commercially producing electricity. This efficient way is penalized by having to operate a complete system at a pressure of 850 psi in order to convert some of the syngas into methanol. The construction of equipment such as coal charging mechanisms, pressure vessels, pumps, valves, piping, etc. must be sized to take such high pressure, yet the bulk of the gas is not converted to methanol but used for power generation which requires much lower pressure. As stated on top of page 5-153, the Once-Through methanol process is integrated with coal gasification and combined cycle electric power generation.
Oxides of Nitrogen (NO.sub.x) which cause a very serious pollution problem to the environment are generated when burning fuels at high temperature. Low Btu gases are known to burn cooler and therefore generate the least NO.sub.x. For example, reference FIG. II from the June, 1983 issue of Power shows the formation of NO.sub.x in parts per million upon combustion of various fuels. It is to be noted that a low Btu gas of 130 Btu per cu. ft. produces at 2000.degree. F. combuster exit temperature about 20 parts per million of NO.sub.x ; whereas, a medium Btu gas (300 Btu's per cu. ft.) which is produced from oxygen blown gasification produces about 280 parts per million at 2000.degree. F. exit temperature. This is a 1400 percent increase in NO.sub.x emission. The instant invention produces a gas of 110 Btu per cu. ft. and therefore should produce less than 20 parts per million. In the case of methanol, it is to be noted that it produces 70 parts per million of NO.sub.x as compared to 230 parts per million for No. 2 fuel oil, or a 329% increase in NO.sub.x emission when using No. 2 oil. Therefore, the thrust of this invention is to produce both gases and liquid fuels which when combusted produce very low levels of NO.sub.x. In addition, the gases produced are desulfurized prior to usage to reduce the sulfur content (mainly SO.sub.2) of the gases to very low levels also.
With the above considerations in view, the main object of this invention is to cogenerate both clean liquid fuels (methanol) and clean electric power from coal in a very cost effective manner.
Another object of the instant invention is to coproduce from coal clean transport fuels like methanol and clean electric power without the need for an oxygen plant for gasification which is very costly, and which produces a relatively polluting gas.
Still another object of this invention is to convert a significant portion of the coal into clean fuel or chemicals.
Further still another object of this invention is to operate the coal gasification at a pressure dictated by the pressure required for combined cycle power generation rather than the much higher pressure required to synthesize gas into methanol.
Further yet another object of this invention is to operate a gasification process that yields a syngas of a ratio of H.sub.2 to CO of 3 to 1 which far exceeds the requirement for feedstock specification for making methanol, this being accomplished without a shift reaction.
Therefore another object of this invention is to efficiently produce clean electric power using a dilute low Btu gas (110 Btu/cu. ft.) produced from coal in order to keep the formation of NO.sub.x (a serious pollutant) at very low levels.
It is still another object of the present invention to provide a method for producing clean-burning methanol as a clean byproduct fuel from coal to substitute polluting petroleum derived fuels.
It is yet another object of the present invention to provide a method that will cogenerate both methanol and electric power from coal with a low capital investment requirement.
It is further yet another object of the present invention to provide a method which makes use of vast domestic coal reserves in a cost effective and environmentally acceptable manner to permit the United States and other coal producing nations to reduce their dependency on imported petroleum.
It is therefore yet another object of the instant invention to provide a method that is specifically configured to convert the volatile matter of the coal feed into a clean feedstock by thermally rearranging the molecular structure of said volatile matter to yield substantially H.sub.2 and CO for the production of chemicals and particularly clean transportation fuels, and also to convert the residual char to a clean fuel gas of such low Btu content as to maintain the creation of NO.sub.x (a serious pollutant) to very low levels upon its combustion.
It is still further another object of the instant invention to provide a method capable of economically coproducing a clean fuel for transport and a clean fuel for power from coal with very low levels of sulfur economically.
These and other objects of the instant invention will become more apparent to those skilled in the art to which this invention pertains and particularly from the following description and appended claims.